Monitoring carrier ethernet networks

ABSTRACT

A Carrier Ethernet network comprises nodes connected by links. The network supports connectionless traffic flows between nodes. A network management system performs a method comprising: determining when an event has occurred in the network which may change bandwidth usage on a link of the network; determining traffic flows which use the link; determining a total bandwidth demand of the traffic flows using the link; and comparing the total bandwidth demand with a bandwidth capacity of the link. Determining when the event has occurred can include at least one of: receiving an alarm indicating a network fault; receiving a topology change indication from a node of the network.

TECHNICAL FIELD

This invention relates to monitoring a Carrier Ethernet (CE) network bya network management system. The Carrier Ethernet network can comprise aProvider Bridge network or a Provider Backbone Bridge (PBB) network.

BACKGROUND

Network operators are replacing conventional time-division multiplexed(TDM) based transport networks with Ethernet based transport networks,which can allow transport of a wide range of traffic types.Ethernet-based networks comprise a network of Ethernet switches.

Technologies which allow the use of Ethernet switches in carriernetworks are Provider Bridge Networks, standardised by the Institute ofElectrical and Electronics Engineers as IEEE 802.1ad, and ProviderBackbone Bridges (PBB), standardised as IEEE 802.1ah. Both of thesetechnologies use Virtual Local Area Network (VLAN) tags to identifytraffic belonging to particular customers. PBB is a technology whichallows for layering of the Ethernet network into customer and providerdomains with complete isolation among their MAC addresses. In this way,a customer's traffic can be carried transparently across a carrier'sEthernet network.

Ethernet networks can be used in a connection-oriented manner usingtechniques such as Multi-Protocol Label Switching Transport Profile(MPLS-TP) or Provider Backbone Bridge Traffic Engineering (PBB-TE), orin a more conventional connectionless manner. When used in aconnectionless manner, the nodes of the Ethernet network use techniquessuch as flooding and learning to establish a loop-free path between thesource and the destination for the configured service. A distributedprotocol such as the Spanning Tree Protocol (STP) can be used. In theevent of a network disturbance, such as a fault or failure of a link ornode, STP will recalculate a loop-free topology. In this specification,the term Ethernet Virtual Connection (EVC) is used to describe a trafficflow across a Carrier Ethernet network between two User NetworkInterfaces (UNI).

From a fulfillment perspective, traffic tagged with the relevant VirtualLocal Area Network (VLAN) tags is configured on all of the ports ofswitches that can be involved in any of the possible packet routes thatthe spanning tree protocol can calculate. This gives provider bridgenetworks a high level of unpredictability in determining where thepackets are really flowing in the network for a given Ethernet serviceat a given time. This is a problem from a service problem detectionperspective as, at the time a fault on the data plane is detected by theNMS/OSS system, the traffic could already have been reverted on anotherroute by the spanning tree protocol. Considering also that Ethernetnetworks are based on statistical multiplexing, it means that it isproblematic to identify if a given Ethernet service is affected bytraffic degradation or traffic disruption.

One of the factors to help deliver carrier-grade performance in anEthernet network is to provide OAM (Operation Administration andMaintenance) capabilities that allow the service provider to detect,isolate and eliminate faults in the network to prevent service trafficdegradation and, in the worst case, service traffic disruption thathighly impact the committed SLA (Service Level Agreement). The IEEE hasstandardised protocols and practices for Operations, Administration, andMaintenance (OAM) for paths through 802.1 bridges and local areanetworks (LANs) in IEEE 802.1ag, “IEEE Standard for Local andMetropolitan Area Networks Virtual Bridged Local Area Networks Amendment5: Connectivity Fault Management”. IEEE 802.1ag defines the concept ofservice OAM, which allows the configuration of Maintenance Entitiesassociated to the Ethernet Service in order to detect if something goeswrong on the Ethernet Service end to end through some connectivityverification procedures. However, service OAM has some limitations.There are limits on the number of Maintenance Entities that areconfigurable in a network element, and therefore this approach may notbe scalable to give a protection for all the configured Ethernetservices.

The present invention seeks to provide an alternative way of monitoringa Carrier Ethernet network.

SUMMARY

An aspect of the present invention provides a method of monitoring aCarrier Ethernet network comprising nodes connected by links. Thenetwork supports connectionless traffic flows between nodes. The methodcomprises, at a network management system, determining when an event hasoccurred in the network which may change bandwidth usage on a link ofthe network. The method further comprises determining traffic flowswhich use the link. The method further comprises determining a totalbandwidth demand of the traffic flows using the link. The method furthercomprises comparing the total bandwidth demand with a bandwidth capacityof the link.

Embodiments of the invention provide an indication when an active linkis becoming a potential bottleneck for the traffic flows using thatlink. When this link is in a bottleneck condition it could raiseproblems in terms of service degradation/disruption for the trafficflows using that link.

Embodiments of the invention provide a complementary, or alternative,way to identify degradation or possible disruption to traffic flows tothe use of Service OAM (IEEE 802.1ag). Embodiments of the invention areparticularly useful in situations where the maximum number of ServiceOAM instance has been reached.

The traffic flows can be Ethernet Virtual Connections (EVC). A trafficflow may be identified by a Virtual Local Area Network Identifier (VLANID), which is also called a VLAN tag. Embodiments of the invention applyto any provider bridge network (PB, PBB) with any version of theSpanning Tree Protocol (i.e. STP, Rapid STP (RSTP) or Multiple STP(MSTP) switched transport infrastructure.

Advantageously, the steps of: determining traffic flows which use thelink; determining a total bandwidth demand of the traffic flows usingthe link; and comparing the total bandwidth demand with a bandwidthcapacity of the link are repeated for a plurality of different links ofthe network.

Advantageously, the method further comprises reporting a linkdegradation if the step of comparing the total bandwidth demand with abandwidth capacity of the link indicates that the total bandwidth demandexceeds the bandwidth capacity of the link.

Advantageously, the step of determining traffic flows which use the linkcomprises querying a node of the network to retrieve active topologyinformation.

Advantageously, the step of determining traffic flows which use the linkcomprises using a store of information which indicates a spanning treeinstance and a traffic flow using that spanning tree.

Advantageously, the step of determining a total bandwidth demand of thetraffic flows using the link comprises using a store of informationwhich indicates traffic flows and parameters of each of the trafficflows.

Advantageously, the step of determining a total bandwidth demand of thetraffic flows using the link comprises: determining a CommittedInformation Rate for each of the traffic flows using the link; and,determining a sum of the Committed Information Rates of the trafficflows using the link.

Advantageously, the step of determining when an event has occurred inthe network which may change bandwidth usage on a link of the networkcomprises at least one of: receiving an alarm indicating a networkfault; receiving a topology change indication from a node of thenetwork; receiving a new root indication from a node of the network.

Advantageously, the step of determining when an event has occurred inthe network which may change bandwidth usage on a link of the networkcomprises: receiving an alarm indicating a network fault; and, waitingfor a time period for nodes of the network to converge on an activetopology.

Another aspect of the invention provides apparatus which is arranged toperform any of the described or claimed steps of the method. Inparticular, an aspect of the invention provides apparatus for use at anetwork management system of a network. The network comprises nodesconnected by links. The network supports connectionless traffic flowsbetween nodes. The apparatus comprises an event monitor which isarranged to determine when an event has occurred in the network whichmay change bandwidth usage on a link of the network. The apparatuscomprises a traffic monitor which is arranged to determine traffic flowswhich use the link. The apparatus comprises a bandwidth monitor which isarranged to determine a total bandwidth demand of the traffic flowsusing the link. The apparatus comprises a comparator which is arrangedto compare the total bandwidth demand with a bandwidth capacity of thelink.

Another aspect of the invention provides a communication networkcomprising any of the described or claimed apparatus.

The functionality described here can be implemented in hardware,software executed by a processing apparatus, or by a combination ofhardware and software. The processing apparatus can comprise a computer,a processor, a state machine, a logic array or any other suitableprocessing apparatus. The processing apparatus can be a general-purposeprocessor which executes software to cause the general-purpose processorto perform the required tasks, or the processing apparatus can bededicated to perform the required functions. Another aspect of theinvention provides machine-readable instructions (software) which, whenexecuted by a processor, perform any of the described methods. Themachine-readable instructions may be stored on an electronic memorydevice, hard disk, optical disk or other machine-readable storagemedium. The machine-readable medium can be a non-transitory medium. Themachine-readable instructions can be downloaded to the storage mediumvia a network connection.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 shows a carrier network and a network management system for thenetwork in accordance with an embodiment of the invention;

FIG. 2 shows a bridge/switch in the network of FIG. 1;

FIGS. 3 and 4 show Ethernet Virtual Connections (EVC) configured on aport or link of the network;

FIG. 5 shows a physical topology of a network and two active topologies(i.e. virtual LANs) which uses the physical topology;

FIGS. 6 to 8 show a method of monitoring degradation of a link of thenetwork;

FIG. 9 shows apparatus for a computer-based implementation of themethod.

DETAILED DESCRIPTION

FIG. 1 shows an example of a communication network 10 in accordance withan embodiment of the present invention. The network 10 comprises aplurality of nodes 11, connected by links 12. The network 10 has aframe/packet-based transport layer, and each node 11 switchesframes/packets between input and output ports of the node based oninformation carried in a header of each frame/packet. In FIG. 1 thenetwork 10 can be a provider network, such as a Provider BackboneBridged Network (PBBN) as defined in IEEE 802.1ah, or a Provider BridgedNetwork, as defined in IEEE 802.1ad. Network 10 can connect to othernetworks 21, 22, such as access networks or customer networks. Nodes 11a, 11 b which connect to other networks are called edge nodes. For aPBBN the edge nodes are called Backbone Edge Bridges (BEB). For aProvider Bridged Network the edge nodes are called Provider Edge Bridges(BEB). The physical layer connecting nodes 11 can be optical, electricalor wireless, and is not important to the invention.

In use, Ethernet Virtual Connections (EVC) are established across thenetwork 10 between User Network Interfaces (UNI). A UNI may be locatedat one of the edge nodes 11 a, 11 b, or at a node of one of the outlyingnetworks 21, 22. One function of a provider network is that it can carrytraffic belonging to different customers. Virtual Local Area Networktags (VLAN) are used to separate traffic of different customers. StackedVLAN (i.e. VLAN in VLAN encapsulation or Q-in-Q) may be used to protectany VLAN tags already used by the customer traffic. In a PBBN, alltraffic is encapsulated at the edge node 11 a, 11 b where the trafficingresses the PBBN, and an additional header identifies the destinationedge node of the PBBN. Additional fields of the new header can separatetraffic belonging to different customers. The encapsulation is removedwhen traffic egresses the PBBN at another of the edge nodes 11 a, 11 b.

The network 10 supports connectionless traffic flows between edge nodes.This means that the path taken by data frames for a particular customeris not fixed, and has not been established in advance by a NMS/OSS or bycontrol plane signalling. The path taken by data frames for a particularcustomer can vary during operation in response to topology changes, suchas faults/failures of links or nodes. This lack of a unpredictability inthe path of a traffic flow also means that the amount of traffic flowingalong a particular link of the network is also unpredictable. This canbe compared to a connection-oriented Ethernet network, where a path isdefined for a traffic flow (i.e. all packets follow the same path) andresources are reserved along that path for the traffic flow, before thatpath is used to carry traffic. Typically, in a connection-orientednetwork an alternative path is also pre-planned for a traffic flow inthe event of a fault, and dedicated or shared resources are reservedalong the alternative path.

FIG. 1 also shows a network management system (NMS) 40 for managing thenetwork 10. The NMS may be centralised at a single node or distributedamong a set of nodes. The NMS comprises a controller 50 comprising a setof functional units or modules 51-54 and a data store 46. The functionalunits comprise: an event monitor 51 which is arranged to determine whenan event has occurred in the network which may change bandwidth usage ona link of the network; a traffic monitor 52 which is arranged todetermine traffic flows which use the link; a bandwidth monitor 53 whichis arranged to determine a total bandwidth demand of the traffic flowsusing the link; and a comparator 54 which is arranged to compare thetotal bandwidth demand with a bandwidth capacity of the link. Thefunctional units 51-54 represent the main functions of the controller50. It will be understood that the functional units 51-54 can beimplemented in various ways, such as software modules executed by one ormore processors, or by hardware. Each functional unit 51-54 may beimplemented by a dedicated processor, or two or more of the multiplefunctional units 51-54 may be implemented by a common processor. Datastore 46 can store:

-   -   (i) physical topology data 47, which is information about how        nodes 11 of the network 10 are interconnected to one another;    -   (ii) active topology data 48, which is information about the        topology used by each virtual connection. Typically, the active        topology is established using a protocol such as the STP;    -   (iii) traffic flow data 49, which is information about        parameters of traffic flows (EVCs) in the network, such as the        bandwidth profile of a traffic flow.

The NMS 40 communicates 30 with nodes 11 in the network using a protocolsuch as the Simple Network Management Protocol (SNMP).

FIG. 2 schematically shows apparatus at one of the nodes 11 of thenetwork 10 of FIG. 1. The node 11 has a plurality of ports 13. Each port13 connects to one of the links 12 which leads to another node 11. Eachlink 12/port 13 may carry multiple Ethernet Virtual Connections (EVC)14. Three EVCs 14 a, 14 b, 14 c are shown as an example. Each of theEVCs 14 a-14 c are carried over the same link 12, but the framescarrying each traffic flow have a different VLAN tag, hence the term“virtual connection”. A port processing unit 15 can perform functionssuch as Medium Access Control (MAC) and buffering. Data received at aport 13 is separated into data frames. Fields in the header of a frame,such as one or more of: the destination address field; source addressfield; VLAN tag are used to determine an egress port 13 for the datapacket (payload). A Forwarding Information Base (FIB) 19 is used todetermine the egress port for each frame. FIB 19 stores informationwhich is used to select the egress port to which a data packet isforwarded. The FIB 19 is populated by periodically performing theSpanning Tree Protocol (STP). The STP determines a loop-free topologyfor forwarding packets to destination nodes. A switching fabric 16interconnects ports 13. A controller 18 controls operation of the node,and performs functions such as look-ups in the FIB 19 and performing theSTP. Controller 18 also communicates 30 with the NMS 40. In accordancewith embodiments of this invention, controller 18 determines when achange has occurred to the active topology (i.e. the path taken byframes) and can notify the NMS 40 when there is a topology change event.Switches exchange Bridge Protocol Data Units (BPDU) to notify activetopology change and configuration changes. Topology change events can besignalled by a topologyChange trap. A bridge that becomes a new rootbridge will send a newRoot trap and all other nodes in the topology willsend the topologyChange trap.

FIGS. 3 and 4 show a set of Ethernet Virtual Connections EVC1, EVC2,EVC3 which all share the same port 13/link 12. As described above, theset of EVCs are identified by different VLAN tags. Each EVC has abandwidth profile. A bandwidth profile is a limit on the rate at whichEthernet frames can traverse an interface. There can be separatebandwidth profiles for frames ingressing to the network and for framesegressing from the network. The Metro Ethernet Forum (MEF) has defined abandwidth profile for Ethernet services which four traffic parameters:Committed Information Rate (CIR); Committed Burst Size (CBS); ExcessInformation Rate (EIR) and Excess Burst Size (EBS). The CommittedInformation Rate (CIR) is the average rate up to which service framesare delivered per the service performance objectives (delay, loss, etc.)Service frames whose average rate is greater than the CIR, or thosewhich send more than CBS bytes are not CIR-conformant and may bediscarded or coloured to indicate non-conformance, depending on whetherthe service frames are EIR-conformant or not. The Excess InformationRate (EIR) specifies the average rate, greater than or equal to the CIR,up to which service frames are delivered without any performanceobjectives. Service frames whose average rate is greater than the EIR,or those which send more than EBS bytes, are not EIR-conformant and maybe discarded or coloured to indicate non-conformance depending on theservice being offered.

As shown in FIG. 3, the bandwidth profiles of services EVC1, EVC2, EVC3may overlap, and may exceed the bandwidth profile of the link over whichthey are carried. Under these circumstances, the link may becomecongested and performance of one or more of EVC1, EVC2, EVC3 may becomebe degraded, and fail to meet the Service Level Agreement (SLA)established between the customer and the service provider.

To help illustrate problems of congestion, FIG. 5 shows an exampletopology for part of the network 10. The network has a physicaltopology. The physical topology represents the way in which nodes 11 areinterconnected by links 12. In FIG. 5, there are six nodes A-F withlinks 12 connecting the nodes in a mesh topology. As described above, aconnectionless Ethernet network will establish a loop-free path fortraffic. The loop-free path is described by a spanning tree. This iscalled the “active topology”, because it is the topology that iscurrently used to forward traffic. The active topology can be definedfor a particular VLAN tag number, or range of VLAN tag numbers. Eachactive topology is called a spanning tree instance. FIG. 5 shows a firstactive topology STP_(—)1 for VLAN IDs 1-20, and a second active topologySTP_(—)2 for VLAN IDs 21-40. The use of multiple spanning trees iscalled Multiple Spanning Tree Protocol (MSTP), and is described in IEEE802.1Q-2005. To explain the active topology STP_(—)1: traffic arrivingat node D and destined for node A is forwarded to node A via the linkD-A; traffic arriving at node D and destined for node B is forwarded tonode E via the link D-E and then to node B via the link E-B; and so on.

FIG. 5 shows an active topology for each range of VLAN IDs and, for eachactive topology, shows the topology which exists before and after afault on link E-B. For VLAN IDs 1-20, STP_(—)1, node B is initiallyreached via the path D-E-B. Following the fault on link E-B, the activetopology has to change as the link E-B is no longer available. In therevised active topology, node B is reached by the path D-E-F-C-B andnode A is reached by the path D-E-F-C-B-A. For VLAN IDs 21-40, STP_(—)2,the active topology also changes after the fault on link E-B. The activetopology is determined by one or more link cost metrics advertised bynodes. Following the fault, it can be seen that the link between nodes Aand B is used by both active topologies. If link A-B does not haveenough capacity to fulfil all the bandwidth requirements of the trafficflows (services) with VLAN IDs (1 to 40), then congestion may occur onlink A-B, and the services will become degraded.

FIG. 6 shows a method performed by the network management system 40. Themethod can respond to an event in the network which could changebandwidth usage on at least one of the links, such as a topology changeevent. An increase in bandwidth usage can cause congestion anddegradation to the quality of service of one or more services carried bythe congested links. The method can be performed in respect of multiplelinks.

At step 101 the method determines if an event has occurred in thenetwork which may change bandwidth usage on a link of the network. If noevent is detected at step 101, the method takes no further action andreturns to step 101 to await an event. The link under consideration atstep 101 and the following steps of the method can be any link of thenetwork. In the example of FIG. 5, an event in the network (the failureof link E-B) may change bandwidth on multiple links of the network.Therefore, it is advantageous to perform the method for multiple links.The event can be one or more of: receiving an alarm indicating a networkfault; receiving a topology change indication from a node of thenetwork; and receiving a new root indication from a node of the network.If one of these events has occurred, the method proceeds to step 102 anddetermines traffic flows which use the link (on which bandwidth usagemay have changed). This step may comprise a step 103 of querying nodesto retrieve active topology information. Advantageously, step 103 onlyqueries those nodes which have reported a topology change at step 101.It has been described how the NMS stores information 47 about thephysical topology of the network. The information 47 about the physicaltopology can define links between ports on the relevant nodes. The NMS40 queries each relevant node, obtaining the Spanning Tree Protocol(STP) status of the ports for the related STP instance. Ports in theforwarding state (i.e. ports which are configured to forwardframes/packets) are selected and, reading the attached links from theNMS database, the active topology is built. This is done for each of theSTP instances in the case of MSTP.

At step 104 the method determines a total bandwidth demand of thetraffic flows using the link. This step can comprise steps 105 ofdetermining CIR for each traffic flow using the link and determining asum of the CIRs of the traffic flows using the link. Advantageously themethod is performed for every port/link of the new active topology.Referring again to the example of FIG. 5, following failure of link E-B,there is a new active topology for STP_(—)1 and STP_(—)2. The method isperformed for every port/link of the new active topologies STP_(—)1,STP_(—)2 to detect any increase in bandwidth usage of the links.

At step 106 the method compares the total bandwidth demand with abandwidth capacity of the link. Advantageously, an EVC is considered asnot degraded if the configured Committed Information Rate (CIR) isavailable and guaranteed in respect to the active topology it isprovisioned on. he CIR is the guaranteed bandwidth specified in the SLAfor the given Ethernet service. EIR (Exceeded Information Rate) andpotential overbooking factor are not considered in the trafficdegradation/disruption detection. If step 106 determines that the totalbandwidth is greater than, or equal to, the bandwidth capacity of thelink, then the method proceeds to step 107. A link congestion/servicedegradation alarm shall be raised. This alarm can be a link alarm or anEVC alarm. The NMS can then, at step 108, take remedial action torelieve the congestion.

When a fault occurs in the network 10, multiple nodes may update theirtopology and therefore step 101 will receive messages from multiplenodes, each indicating that a topology change has occurred.Advantageously, the NMS is be able to correlate the topology changeevents to the relevant Spanning Tree instance to avoid performing thealgorithm more than one time for the same Spanning Tree instancetopology change. FIG. 7 shows an alternative form of step 101. Asbefore, step 101 determines if an event has occurred in the networkwhich may change bandwidth usage on a link of the network. Step 101Aawaits receipt of a notification, from a node, of a link failure. A linkfailure can be notified by sending an alarm to the NMS. The alarm canidentify a particular port of a node. If a link failure alarm isreceived at step 101A, this indicates that a link has failed and thatnodes may attempt to converge to a new active topology which avoids thefailed link. The method proceeds to step 101B and waits for a timeperiod. The time period can be configurable. Advantageously, the timeperiod is sufficiently long to allow a set of nodes to converge to a newactive topology using the STP. Step 101B serves as a time filter. Duringthis time period, step 101B collects topology change notifications fromnodes. When the time period of step 101B has expired, the methodproceeds to step 102 of FIG. 6, and proceeds in the same manner aspreviously described. Advantageously, steps 102, 103 query only thosenodes which reported a topology change at step 101B.

FIG. 8 shows the method in more detail. In addition to determining ifcongestion has occurred, the method can minimise the overall networkpower consumption by identifying any ports that are not carryingtraffic. The source (typically a laser in the case of optical Ethernet)can be turned off, and therefore does not consume power but at a powerlevel that is enough to be restarted quickly to react to an activationrequest.

The service degradation indication shall be in charge of the OSS systemthat shall be able to provide the following services:

-   -   Physical Topology management    -   Spanning tree active topology monitoring    -   EVC configuration (VLAN registration on ports)        In case of MSTP, the OSS System shall be able to store an        association between the Multiple Spanning Tree Instance (MSTI)        and the configured EVCs (VLANs) over it. We can consider a        single MSTI instance in case of RSTP based networks to unify the        model. Every MSTI represent a traffic layer where the sum of all        the EVC CIR shall be always available on every links of the        related active topology. Every time there is a topology change        in an MSTI due to a Fault on the physical network, its active        topology shall be rebuilt and for every link in the MSTI the sum        of the CIR of the configured EVCs shall be available. Every MSTI        change shall be processed in parallel to provide fast        degradation detection.

Referring again to FIG. 1, controller 50 comprises functional modules51-54 for performing the method of FIGS. 6 to 8. Controller 50comprises: a module 51 which is arranged to determine when an event hasoccurred in the network which may change bandwidth usage on a link ofthe network; a module 52 which is arranged to determine traffic flowswhich use the link; a module 53 which is arranged to determine a totalbandwidth demand of the traffic flows using the link; and a module 54which is arranged to compare the total bandwidth demand with a bandwidthcapacity of the link.

FIG. 9 shows an exemplary processing apparatus 130 which may beimplemented as any form of a computing and/or electronic device, and inwhich embodiments of the system and methods described above may beimplemented. Processing apparatus 130 can be provided at the NMS 40.Processing apparatus may implement the method shown in any of FIGS. 6 to8. Processing apparatus 130 comprises one or more processors 131 whichmay be microprocessors, controllers or any other suitable type ofprocessors for executing instructions to control the operation of thedevice. The processor 131 is connected to other components of the devicevia one or more buses 136. Processor-executable instructions 133 may beprovided using any computer-readable media, such as memory 132. Theprocessor-executable instructions 133 can comprise instructions forimplementing the functional modules 51-54. The memory 132 is of anysuitable type such as read-only memory (ROM), random access memory(RAM), a storage device of any type such as a magnetic or opticalstorage device. Additional memory 134 can be provided to store data 135used by the processor 131. The processing apparatus 130 comprises one ormore network interfaces 138 for interfacing with other network entities,such as nodes 11 of the network 10.

Modifications and other embodiments of the disclosed invention will cometo mind to one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed and that modifications andother embodiments are intended to be included within the scope of thisdisclosure. Although specific terms may be employed herein, they areused in a generic and descriptive sense only and not for purposes oflimitation.

1. A method of monitoring a Carrier Ethernet network comprising nodesconnected by links, the network supporting connectionless traffic flowsbetween nodes, the method comprising, at a network management system:determining when an event has occurred in the network which may changebandwidth usage on a link of the network; determining traffic flowswhich use the link; determining a total bandwidth demand of the trafficflows using the link; and comparing the total bandwidth demand with abandwidth capacity of the link.
 2. A method according to claim 1 furthercomprising reporting a link degradation if the step of comparing thetotal bandwidth demand with a bandwidth capacity of the link indicatesthat the total bandwidth demand exceeds the bandwidth capacity of thelink.
 3. A method according to claim 1 wherein the step of determiningtraffic flows which use the link comprises querying a node of thenetwork to retrieve active topology information.
 4. A method accordingto claim 1 wherein the step of determining traffic flows which use thelink comprises using a store of information which indicates a spanningtree instance and a traffic flow using that spanning tree.
 5. A methodaccording to claim 1 wherein the step of determining a total bandwidthdemand of the traffic flows using the link comprises using a store ofinformation which indicates traffic flows and parameters of each of thetraffic flows.
 6. A method according to claim 1 wherein the step ofdetermining a total bandwidth demand of the traffic flows using the linkcomprises: determining a Committed Information Rate for each of thetraffic flows using the link; and determining a sum of the CommittedInformation Rates of the traffic flows using the link.
 7. A methodaccording to claim 1 wherein the step of determining when an event hasoccurred in the network which may change bandwidth usage on a link ofthe network comprises at least one of: receiving an alarm indicating anetwork fault; receiving a topology change indication from a node of thenetwork; and receiving a new root indication from a node of the network.8. A method according to claim 1 wherein the step of determining when anevent has occurred in the network which may change bandwidth usage on alink of the network comprises: receiving an alarm indicating a networkfault; and waiting for a time period for nodes of the network toconverge on an active topology.
 9. A method according to claim 1,comprising repeating for a plurality of different links of the networkthe steps of: determining traffic flows which use the link, determiningthe total bandwidth demand of the traffic flows using the link, andcomparing the total bandwidth demand with the bandwidth capacity of thelink.
 10. An apparatus for use at a network management system of anetwork comprising nodes connected by links, the network supportingconnectionless traffic flows between nodes, the apparatus comprising: anevent monitor which is arranged to determine when an event has occurredin the network which may change bandwidth usage on a link of thenetwork; a traffic monitor which is arranged to determine traffic flowswhich use the link; a bandwidth monitor which is arranged to determine atotal bandwidth demand of the traffic flows using the link; and acomparator which is arranged to compare the total bandwidth demand witha bandwidth capacity of the link.
 11. The apparatus according to claim10 wherein the comparator is further arranged to report a linkdegradation if the total bandwidth demand exceeds the bandwidth capacityof the link.
 12. The apparatus according to claim 10 wherein the trafficmonitor is arranged to query nodes of the network to retrieve activetopology information.
 13. The apparatus according to claim 10 furthercomprising a store which is arranged to store information whichindicates traffic flows and parameters of each of the traffic flows andthe bandwidth monitor is arranged to use the store.
 14. The apparatus ofclaim 10, wherein the apparatus is an element in a Carrier Ethernetcommunication network comprising a plurality of nodes connected bylinks, the network supporting connectionless traffic flows betweennodes.
 15. A non-transitory machine-readable medium carryinginstructions which, when executed by a processor, cause the processor toperform a method of monitoring a Carrier Ethernet network comprisingnodes connected by links, the network supporting connectionless trafficflows between nodes, the method comprising, at a network managementsystem: determining when an event has occurred in the network which maychange bandwidth usage on a link of the network; determining trafficflows which use the link; determining a total bandwidth demand of thetraffic flows using the link; and comparing the total bandwidth demandwith a bandwidth capacity of the link.